Microlithography is used for producing microstructured components, such as for example integrated circuits. The microlithography process is carried out with a lithography apparatus including an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography systems, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, are used instead of—as previously—refractive optical units, that is to say lens elements.
The mirrors may for example be fastened to a supporting frame (force frame) and be designed as at least partially manipulable, in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the pm range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be compensated for.
For mounting the mirrors on the supporting frame, usually gravity compensation devices on the basis of permanent magnets (magnetic gravity compensators) are used, as described for example in DE 10 2011 088 735 A1. The compensation force generated by such a gravity compensation device acts counter to the weight of the mirror, and it corresponds substantially to the weight in terms of its absolute value.
By contrast, the movement of a respective mirror—in particular also in the vertical direction—is actively controlled by way of so-called Lorentz actuators. Such a Lorentz actuator includes an energizable coil and, at a distance from it, a permanent magnet. These together generate an adjustable magnetic force for controlling the movement of the respective mirror. By way of example, such Lorentz actuators are described in DE 10 2011 004 607.
Both the gravity compensation devices and the Lorenz actuators contact the corresponding mirror via bearing devices. Here, it was found to be problematic that the numerical aperture of EUV projection systems, in particular, increasingly increases, leading to larger mirror surfaces and hence higher mirror masses. In turn, this means a higher mechanical load on the aforementioned bearing devices. This could be accounted for by elements of the bearing device having a more stable embodiment, in particular with greater material cross sections. However, this may be disadvantageous, in turn, in view of parasitic forces and the only restricted installation space available.